XmlRpcPlugin: Reworked to normalize output, so that any method can just return regular Python types as used in Trac - typically unicode, datetime, None. It simplifies many method implementations by not having to convert timestamps, check for stray None objects and similar.

Bumped version as these internal changes may require changes to other plugins that provide xmlrpc methods.

XmlRpcPlugin: Adding RPC to systeminfo dictionary, so that the plugin version information will show up under 'About' (for administrators at least). Please include the table of software versions when reporting issues.

XmlRpcPlugin: Changes to datetime handling, by converting any timestamps to Python datetime before arguments are passed to methods. Like Trac 0.11, all code should use and expect regular datetime.datetime objects.

The rework of code to transform input, should also make it easier to solve other issues related to input.